1. Field of the Invention
The present invention relates to a developer transporting amount controlling member, a developing apparatus, an imaging apparatus, and a method for exchanging a developing unit.
2. Description of the Related Art
An imaging apparatus such as a printer or a copier may include a developing apparatus and a so-called photoconductor drum as a recording unit. In an imaging apparatus having such a configuration, an image may be recorded by charging the photoconductor drum at a predetermined potential and exposing light on the photoconductor drum according to recording image information so that a latent image may be formed on the photoconductor drum. Then, toner as an image visualizing agent may be supplied from the developing apparatus to transform the electrostatic latent image on the photoconductor drum into a visible image (toner image). Then, the visible image may be transferred and fixed to a recording medium to be printed.
The developing apparatus as is described above may include a developer accommodating container, a developer transporting unit, a developing roller as a developing unit, a developer transporting amount controlling member, and a toner supplying unit, for example.
The developer may be a two-component developer made up of toner and magnetic powder referred to as “carrier” used for charging and transporting the toner. The toner and the carrier may be combined at a predetermined combination ratio, for example. Such a developer is stirred by the developer transporting unit so that the toner and the carrier contained in the developer come into frictional contact with each other. In turn, the toner may be charged to a predetermined potential to be adhered to the carrier.
The toner adhered to the carrier is guided by a developing roller that has plural magnets arranged in its interior and a rotating sleeve roller arranged at its periphery. The toner is attracted to the surface of the developing roller by the magnetic force of the magnets arranged inside the developing roller, and the toner adhered to the surface of the developing roller is transported by the rotation of the sleeve roller to pass through a gap created between the developing roller and a developer transporting amount controlling member referred to as “doctor blade” that is arranged opposite the developing roller and is configured to control the amount of developer transported by the sleeve roller to a predetermined amount.
The developer controlled by the doctor blade is transported to a position opposite the photoconductor drum through rotation of the sleeve roller so that the toner comes into contact with the photoconductor drum. At this point, a bias voltage (referred to as “developing bias” hereinafter) is applied to the developing roller, and an electric field is generated by interaction with the static latent image formed on the photoconductor drum. In turn, the charged toner is adhered to an image forming position on the photoconductor drum by the generated electric field so that the electrostatic latent image may be developed on the photoconductor drum.
It is noted that one of plural magnetic poles is arranged at the position of a developing portion of the developing roller opposite the photoconductor drum where the developer adhered to the developing roller comes into contact with the photoconductor drum to develop an image. In this case, the line of magnetic force of the magnetic pole is directed away from the developing roller. Accordingly, the carrier extends along the direction of the line of magnetic force away from the developing roller and toward the photoconductor drum to form magnetic bead chains that are suspended from the surface of the sleeve roller of the developing roller, such magnetic bead chains forming a so-called magnetic brush.
In recent years and continuing, with the growing demand for higher image quality, techniques are being developed for reducing the particle size of the carrier included in the developer. By reducing the particle size of the carrier, the beads of the developer coming into frictional contact with the surface of the photoconductor drum may become finer, and the electrostatic latent image formed on the photoconductor drum may be developed more accurately so that a fine image with reduced roughness may be generated.
However, when the particle size of the carrier is reduced, the magnetic force for holding the carrier at the developing apparatus may be reduced so that carrier jumping may easily occur. In order to prevent jumping of such small carrier particles, the magnetic force acting on the carrier particles has to be adequate for maintaining the carrier particles adhered to the tip of the magnetic brush.
In this respect, the bead chain length of the magnetic brush is preferably reduced. However, when the bead chain length of the magnetic brush is reduced, a developing gap corresponding to the closest distance between the developing roller and the photoconductor drum, and a doctor gap corresponding to the closest distance between the doctor blade and the developing roller have to be reduced as well.
When the doctor gap is arranged to be narrow, variations may occur in the doctor gap due to deviations in the processing accuracy or assembling accuracy of device components, and it may be difficult to adjust the transporting amount of developer passing through the doctor gap to a fixed amount. Also, when the developer transporting amount is increased, the mechanical scratching force of the magnetic brush at the developing portion may be increased so that scratch marks of the carrier may be formed on the developed image, for example. On the other hand, when the developer transporting amount is less than a desirable amount, the image density may not reach a target value, or unevenness in the image density may occur. Accordingly, measures need to be taken to reduce variations in the transporting amount of the developer on the developing roller with respect to the variations in the doctor gap.
Also, when the doctor gap is arranged to be narrow, stress applied to the developer passing through the doctor gap may be increased so that high image quality printing may not be maintained for a long period. In this respect, stress applied to the developer at the doctor gap is preferably decreased.
It is noted that a technique is disclosed in Japanese Laid-Open Patent Publication No. 2005-91953 for reducing the developer transporting amount while securing a relatively wide doctor gap. According to the disclosed technique, a magnetic member is fixed to a nonmagnetic doctor blade, and the doctor blade is disposed opposite the magnetic pole of the developing roller to control the developer transporting amount.
With such a configuration, the developer transporting amount may be controlled with the magnetic brush extending in the direction of the line of magnetic force so that the amount of developer transported to the developing portion may be controlled to be a relatively small amount even when the doctor gap is relatively wide. Also, the doctor gap width may be flexibly adjusted to be within a wider dimension range for obtaining the same developer transporting amount so that variations in the developer transporting amount may be reduced with respect to variations in the doctor gap and the stress applied to the developer may be reduced.
Japanese Laid-Open Patent Publication No. 2005-91953 also discloses a technique for countering unevenness of the developer transporting amount with respect to the developing roller axis direction due to bending or deformation of the doctor blade as a result of receiving a force from the developer upon controlling the developer transporting amount. According to the disclosed technique, the shape of the magnetic member is adjusted along the developing roller axis direction so that the magnetic field at the center portion of the developing roller may be strengthened.
Also, other configurations of the doctor blade have been developed for controlling the developer transporting amount including a doctor blade having a tip portion made of magnetic material for controlling the layer thickness of developer and other portions made of nonmagnetic material, a doctor blade having a magnetic member attached to a nonmagnetic member, the doctor blade having an interleaved structure in which a resin member made of resin material such as PTFE (polytetrafluoroethylene) is arranged on both sides of a magnetic member (e.g., see Japanese Laid-Open Patent Publication No. 57-136671).
It is noted that there is a growing demand for the development of techniques for achieving high image quality even in an imaging apparatus adapted for high speed image formation (e.g., image printing). In such a high speed imaging apparatus, the peripheral speed of the developing roller has to be increased in order to secure an adequate image (printing) density. For example, the developing roller may have to be operated at a peripheral speed of at least 1.0 m/s.
However, in such a high speed apparatus, the doctor blade and the surface of the developing roller may wear out as a result of friction with the developer upon controlling the developer transporting amount.
When the doctor blade wears out, the doctor gap may widen and the developer transporting amount may change as a result. Also, a hard material that can adequately resist wear has to be used for the portion of the doctor blade that controls the layer thickness of the developer. That is, soft magnetic material that is normally used for the magnetic member of the doctor blade such as soft magnetic iron may not be used since such material does not have adequate resistance to wear.
The surface of the sleeve roller of the developing roller is roughened through sand blasting or thermal spraying in order to stably transport the developer. However, when the roughness of the surface is reduced by wear, the developer may slide across the surface so that stable transportation of the developer may be hampered. Therefore, the developer roller has to be exchanged when its surface is worn out.
When a magnetic material is used for the doctor blade, carrier particles adhere to the doctor blade due to the magnetic force of the magnetic material. Such carrier particles adhered to the doctor blade may be an obstacle to measuring and adjusting the doctor gap after exchanging the developing roller.